28moreff_fc.cpp

#include <iostream>
using namespace std;

class Y;
// forward declaration is important if a class is passed as an argument before it is even created
class X
{
    int data;
    friend void swap(X &, Y &); //pass by reference
    friend void display(X , Y);
public:
    void setValue(int value)
    {
        data = value;
    }
    friend void add(X, Y); // here, in a friend function Y is declared as an argument but Y do not exist yet
};
class Y
{
    int data;
    friend void swap(X &, Y &);
    friend void display(X , Y);
public:
    void setValue(int value)
    {
        data = value;
    }
    friend void add(X, Y); // since Y has been declared, here there will be no error accessing data of Y
};

void add(X o1, Y o2)
{
    cout << "Sum of the data of X and Y is " << o1.data + o2.data;
}

void swap(X &o1, Y &o2) //pass by reference
{
    int temp = o1.data;
    o1.data = o2.data;
    o2.data = temp;
}

void display(X o1, Y o2)
{
    cout<<endl<<"a = "<<o1.data<<"  b = "<<o2.data<<endl;
}
int main()
{
    X a;
    Y b;
    a.setValue(6);
    b.setValue(3);

    add(a, b);

    cout<<endl<<"Before swap : "<<endl;
    display(a, b);
    swap(a, b);
    cout<<endl<<"After swap : "<<endl;
    display(a, b);
    return 0;
}